This invention relates to the spraying of paint, preferrably to an automatic system for spraying articles on a moving conveyor line wherein periodic or random changes of color are necessary. More specifically, the invention relates to a paint spraying apparatus for proportioning and mixing two paint components together for delivery to a spray gun.
One of the developments of modern paint technology is a liquid urethane paint having a color component and a catalyst component, each of which are relatively stable when isolated but which become highly reactive when mixed together. One of the consequences of the highly reactive mixture is that the pot life of the mixture becomes very short, and the paint mixture begins curing in a relatively few minutes. Because of this reactive characteristic the paint components must be mixed as close to the point of application as possible, and must not be allowed to remain in a paint line or spray apparatus for more than a few minutes without purging with a solvent for cleaning out the residue. This requires that special paint handling, mixing, and solvent purging equipment be developed especially for use with this type of paint.
A further characteristic of highly reactive urethane two-component paints is that the ratio of the colorant component and the catalyst component requires very strict regulation for proper and consistent color control on the finished product. Ratio tolerances having accuracies to .+-. 2% are desired in order to maintain uniform color consistency, for variations in ratio will cause the paint curing process to vary on the finished product. Furthermore, the absolute value of the ratio of colorant to catalyst may vary from one color mixture to another, requiring that a new ratio setting be made for different color selections, and once made, that this ratio be held to close tolerance levels.
While it is possible in the simplest case to control the ratio of two fluids by merely adjusting a valve setting in the respective fluid lines, this is not an adequate solution when the fluids being controlled are subjected to flow variant effects such as changes in viscosity, pressure and temperature, as well as changes in the flow restricting characteristics of apparatus downstream from the valve. For example, the flow rate in a paint spray system may be at least crudely controlled by inserting a valve in the paint line. However, variations in upstream paint pressure will affect flow rate in the line as will variations in temperature and batch characteristics of the paint. A gradual buildup or deposit of residue in the paint lines may change the flow rate. The paint spray gun itself affects flow rate, in that temporary obstructions in the paint spray orifice may impede flow, and gradual wear of the orifice may increase flow over time. All of the aforementioned variables must be either compensated for or controlled in an automatic paint spray system which utilizes two-component urethane paint.